FM-CW radar apparatus

ABSTRACT

The invention relates to an FM-CW radar apparatus where the frequency sweep duration (R) corresponding with the transmitted signal is increased to obtain a mixing signal which is operational in a wider frequency range than the frequency sweep of the transmitted signal. This has the effect that processing of echo signals of sufficient length can be realized, also from objects which are located near the limits of the radar range.

The invention relates to an FM-CW radar apparatus provided with atransmitter for generating transmitter signals at a particular frequencysweep rate (Δf/Δt) across a (first) frequency interval (f₀, f₀ +Δf), andwith a receiver comprising a mixer using mixing signals for demodulationof echo signals obtained from the transmitted signals, and comprising aDFT unit, which is provided with terminals suitable for the reception ofdigitised and sampled signals and which unit is designed to providetarget distance data related to the heterodyne frequency and referringto the signals supplied.

Such a radar apparatus is known from the patent U.S. No. 4,568,938. Theradar apparatus disclosed in this patent is an altimeter, particularlyfor measurement of the distance to objects in its vicinity. However, theradar apparatus is not suitable for measuring the distance to an objectclose to the limits of the range of this radar apparatus, since thesignal duration of the modulated echo signal of the object decreaseswithin the corresponding pulse repetition time when the object isfurther away.

The present invention is based on the idea of increasing the frequencysweep duration of the mixing signal which is operational in a widerfrequency range, without increasing the frequency sweep duration of thetransmitter signal. Thus the objective is achieved, i.e. processing echosignals of sufficient length from objects which are located near thelimits of the radar range.

According to the invention, a radar apparatus of the type described inthe opening paragraph has a transmitter suitable for generating the saidmixing signals at the said frequency sweep rate (Δf/Δt) across a secondfrequency interval (f₁, f₁ +k·Δf) with k>1, which is longer than thefirst frequency interval.

The invention will now be described in more detail with reference to theaccompanying figures, of which:

FIG. 1 shows a timing diagram of the signals occurring in an FM-CW radarapparatus;

FIG. 2 shows a timing diagram of the transmitter signals occurring in anFM-CW radar apparatus, with accompanying time sequences for the variousrelated processing times;

FIG. 3 is a block diagram of a first embodiment of an FM-CW radarapparatus;

FIG. 4 is a block diagram of another embodiment of an FM-CW radarapparatus.

The diagram of FIG. 1 shows the variation with time of transmittingfrequency f_(z) of an FM-CW radar apparatus. The transmitterrepetitively covers a frequency interval (f₀, f₀ +Δf) with a frequencyf_(r) =T⁻¹ at a constant frequency sweep rate (df/dt). The diagram showsthat, for a target located in the radar's proximity (target distance R₁=1/2cT₁), echo signal "a" is received during a considerable part of thelistening time T. When echo signal "a" is demodulated with a mixingsignal, which can be considered as a replica of the current transmittersignal, the heterodyne signal, also called beat signal, is obtained,having a freqency f_(B).sup.(1). This beat signal is long enough toprovide sufficient information on the target distance. This is not sofor an echo signal "b" generated by an object at a target distance R₂=1/2cT₂ near the time limits of the radar range. Echo signal "b" showsthat it is only during a short period (T₂, T) within the listening time(0, T) mixed with the then present mixing signal to obtain the beatsignal having a frequency f_(B).sup.(2). Only if generation of themixing signal to higher frequencies f_(m) with f_(m) >f₀ +Δf, at thesame frequency sweep rate, would be continued would it be possible toreceive longer echo signals from objects at greater distances, and thusto obtain a beat signal of sufficient length. Since for the generationof the transmitter signal the transmitter can only operate in the range(f₀, f₀ +Δf), it is impossible to actually derive such a mixing signalfrom the transmitted signal. By increasing the frequency sweep lengthfor the generation of the mixing signal required for demodulation of theecho signal, but not for the generation of the transmitter signal, whilemaintaining the frequency sweep rate belonging to the transmittersignal, a beat signal of sufficient length can be obtained within thelistening time from an echo signal from a far away target. The totalperiod of time during which generation of the mixing signal should takeplace is preferably 2T, but may be longer or shorter if required.

In the following descriptions of the drawings, a time of 2T is assumedfor the generation of a mixing signal. Although the actual generation ofthe transmitter signals takes up a period of time T, the reception ofthe echo signals and thus the processing of these echo signals by a DFTunit takes up a period of time 2T, and will take place in time interval(0, 2T).

According to an advantageous embodiment of the invention, an additionalmeasure, i.e. introduction of dispersive delay means, may be applied,where after generation of the transmitter signals within the timeinterval (0, T) indicated with E₁, the echo signals received within thetime interval (0, 2T) indicated with R₁ after demodulation undergo afrequency dependent delay to such an extent that all resulting signalsare concentrated within a time interval (T+τ, 2T+τ) indicated with P₁and thus within a period of time T, as indicated in FIG. 2. The resultwill be that a DFT processing period T is added to transmitting time T,where the position of time interval P₁ with respect to time interval R₁is entirely dependent on the type of dispersive delay means applied.This in fact amounts to the realisation of a type of "pulse compression"in a CW radar, which in pulse-doppler radars entails known advantages.With this type of "pulse compression" the signal-to-noise ratio isimproved. This compression can be further increased by generating themixing signal during a time interval k·T (k>2 and KεN), while while thedispersive delay means again concentrate the received signal within atime length T.

Generally according to the invention the mixing signal is generatedduring k.T (k>1 and kεN) seconds, so that time interval R₁ is longerthan time intervall E₁. Additionally, the introduction of dispersivedelay means can achieve a type of pulse compression by concentrating thereceived signal within time interval P₁, where time interval P₁ isshorter than time interval R₁. According to the special embodiment asdescribed above, however, time interval P₁ has the same length as timeinterval E₁, which should not be considered a restriction of theinvention.

FIG. 3 illustrates an embodiment of an FM-CW radar apparatus accordingto the invention. This block diagram shows a transmitting generator 1,which is suitable for the generation of signals within a frequency range(f₀, f₀ +2Δf) at a fixed frequency sweep rate df/dt, in such a way thatduring subsequent frequency sweep periods of a duration T, the frequencyrange is covered from f₀ to f₀ +Δf. Via a closed switch 2 these signalsare supplied to a power amplifier 3 to obtain transmitter signals, whichare transmitted via transmitting antenna 4. The signals produced bytransmitter generator 1 are also supplied to a mixer 6 via a decoupler(or duplexer) 5. Furthermore, the echo signals obtained via receivingantenna 7 are, after amplification in amplifier 8 of the LNTA type, alsosupplied to mixer 6. To achieve the required operation, wheretransmitter signals are only transmitted during time period T within afrequency range (f₀, f₀ +Δf), but where the mixing signals are generatedduring a time period 2T within a frequency range (f₀, f₀ +2Δf),transmitter generator 1 is suitable to maintain the frequency sweep inthe subsequent time period T at the same frequency sweep rate df/dtacross the next frequency interval (f₀ +Δf, f₀ +2Δf), and switch 2 isopened after time period T. The beat signals obtained by demodulation ofthe echo signals with the transmitting signals, are amplified in m.f.amplifier 9 and filtered there as a result of the permitted band width.The output signals of amplifier m.f. 9 are subsequently supplied to adispersive delay line 10, in which the frequency signals having thelowest beat frequency are delayed by a time period T with respect tothose having the highest beat frequency. The output signals of thedispersive delay line 10, which as a result have shifted and beenconcentrated within the same interval T, are sampled and digitised in ananalogue digital converter 11 and subsequently supplied to a DFT signalprocessing unit 12. Because to each frequency value of the target echosignal a target distance can be allocated and this is also true for thefrequency value of the beat signal, each frequency channel of the DFTsignal processing unit will represent a particular distance interval, towhich the measured target belongs. It is furthermore possible toexchange the sequence of delay line 10 and analogue-digital converter11, as illustrated in the inset of FIG. 3.

Because generator 1 is engaged in the generation of the mixing signalfor the mixer during the entire period 2T, it is not possible tomaintain the pulse repetition frequency at the value f_(r) =T⁻¹ in theembodiment shown in FIG. 1. However, this will be possible in theembodiment of an FM-CW radar apparatus as illustrated in FIG. 4, which,as opposed to the radar apparatus represented in FIG. 3, comprisesbesides the first transmitter generator 1, a second transmittergenerator 13 of the same type, as well as a bipolar switching unit 14instead of switch 2 for alternatively connecting transmitter generators1 and 13 to power amplifier 3, which is to take place using a timecontrol unit 15. By means of this time control unit, transmittergenerators 1 and 13 are, alternately and in a rhythm determined by therepetition frequency f=T⁻¹, continuously reset to the lowest value f₀ ofthe corresponding frequency range, while switch 14 is switched by timecontrol unit 15 in such a way that a transmitter signal having afrequency varying within frequency range (f₀, f₀ +Δf) is repetitiously(f=T⁻¹) supplied to power amplifier 3. Decoupling of a small amount ofsignal energy from the output lines of transmitter generators 1 and 13by means of duplexers 5 and 16 respectively and subsequent concentrationof this signal energy by means of a duplexer 17, provides mixer 6 withthe required mixing signals for demodulation of the received echosignals.

I claim:
 1. FM-CW radar apparatus comprising:transmitter means forgenerating transmitter signals at a frequency sweep rate (Δf/Δt) acrossa frequency interval (f₀, f₀ +Δf); receiver means including mixer meansusing mixing signals for demodulation of echo signals obtained from thetransmitted signals; DFT means including terminals suitable for thereception of digitised and sampled signals and which DFT means providestarget distance data; and means for generating said mixing signals atsaid frequency sweep rate (Δf/Δt) across a second frequency interval(f₁, f₁ =k·Δf), which is longer than the first frequency interval (k>1).2. FM-CW radar apparatus as claimed in claim 1 comprising dispersivedelay means connected between the mixer and the DFT means for delayingthe demodulated echo signals in dependency of the signal frequency. 3.FM-CW radar apparatus as claimed in claim 1, wherein the secondfrequency interval (f₁, f₁ +k·Δf) is twice as long as the firstfrequency interval (k=2).
 4. FM-CW radar apparatus as claimed in claim1, wherein the lowest frequency values occuring in both frequencyintervals are identical.
 5. FM-CW apparatus as claimed in claim 1,comprising sampling and digitisation means connected between the mixerand the delay means.
 6. FM-CW radar apparatus as claimed in claim 4,comprising sampling and digitisation means connected between the delaymeans and the DFT unit.
 7. FM-CW radar apparatus as claimed in any ofclaims 1-6, comprising a first and a second transmitter generator, bothgenerators being suitable for the execution of a frequency sweep acrossthe second frequency interval; and switching means:for alternatelyinterconnecting one of the transmitter generators with the antenna meansand for simultaneously resetting to the initial position the transmittergenerator then interconnected with the antenna means; and combiner meansconnected to both transmitter generators, for obtaining the mixingsignals.